Clinical overview
Obstetric anal sphincter injuries (OASIS) — third- and fourth-degree perineal tears — are among the most consequential of all delivery complications, because the injury that is missed in the delivery room becomes the woman who returns months later with faecal urgency, flatal incontinence, perineal pain or a rectovaginal fistula. The sphincter complex governs continence; a tear that is not recognised and repaired correctly the first time is far harder to salvage at secondary surgery. For the FCOG(SA) registrar this objective is therefore a discipline of attention: every vaginal birth ends with a deliberate, systematic perineal and rectal examination, because the single greatest determinant of a good outcome is whether the injury is detected at all.
In South Africa, where many vaginal births occur at district hospitals and Midwife Obstetric Units, the registrar carries a dual responsibility — to repair correctly when the woman is in front of you, and to build the local culture (training, protocols, audit) that ensures the midwife conducting an unsupervised delivery recognises and refers an OASIS rather than suturing it as a second-degree tear. OASIS sits alongside perineal-protection and instrumental-delivery as a triad of intrapartum technique that the exam expects you to integrate. The condition is not, in the immediate sense, life-threatening, but it is a "must-not-miss" and a major cause of long-term maternal morbidity and litigation.
Core knowledge
Anatomy of the sphincter complex
Continence depends on two muscular rings around the anal canal. The external anal sphincter (EAS) is striated, voluntary, tonically active and supplied by the pudendal nerve; it provides the squeeze pressure that defers defaecation. The internal anal sphincter (IAS) is smooth muscle, involuntary, a downward continuation of the rectal circular muscle, and provides the majority of resting anal tone. The IAS is the structure most relevant to passive leakage and flatal incontinence, and it is the layer most often overlooked at primary repair. Relate this to genital-anatomy when revising the pelvic floor.
Classification (Sultan / RCOG, adopted by NICE and SA practice)
The internationally accepted classification grades by depth into the sphincter complex:
| Degree | Structures involved |
|---|---|
| First | Perineal skin / vaginal epithelium only |
| Second | Perineal muscles, sphincter intact (includes episiotomy) |
| 3a | < 50% of EAS thickness torn |
| 3b | > 50% of EAS thickness torn |
| 3c | EAS and IAS torn |
| Fourth | EAS + IAS + anorectal/anal mucosa torn |
A "buttonhole" tear of the rectal mucosa with an intact sphincter is a separate entity that must be recognised and repaired in its own right, as it predisposes to fistula. Grading every OASIS precisely matters because it drives the repair technique and the counselling about future delivery.
Figure J10.1 — OASIS grading map showing Sultan/RCOG depth-based classification from first-degree tears through 3a/3b/3c and fourth-degree injury, with IAS, EAS, anal mucosa and buttonhole pitfalls highlighted.
Mechanism of injury
The sphincter is torn when the descending presenting part overstretches and lacerates the posterior perineum as the head (or, in breech, the after-coming head/shoulders) distends the introitus. The forces are greatest at crowning, when the perineum is maximally stretched over the largest diameter. Anything that increases the rate or magnitude of that stretch, or that reduces perineal compliance, increases tear risk: a precipitate uncontrolled delivery, a large fetal head, occipito-posterior position presenting a larger diameter, instrumental traction (particularly forceps), and shoulder dystocia. A midline episiotomy propagates directly toward the anus and is strongly associated with sphincter extension — which is precisely why South African and UK practice favours the mediolateral cut, angled away from the midline (target ~60° from the midline at the point of incision, when the perineum is distended). This mechanistic understanding connects directly to the prevention bundle below and to perineal-protection.
Risk factors
Strong, recurring associations in the literature (cite cautiously where not line-itemed in a guideline you can name):
- Nulliparity — the single most consistent independent risk factor.
- Instrumental delivery, forceps > vacuum; mid-cavity and rotational instrumentation highest. See instrumental-delivery.
- Fetal macrosomia (classically birthweight > 4 kg). See macrosomia.
- Shoulder dystocia.
- Occipito-posterior position and prolonged second stage.
- Midline episiotomy; and the absence of a protective mediolateral episiotomy in a high-risk instrumental delivery.
- Previous OASIS (recurrence risk in a subsequent vaginal birth).
- Asian ethnicity is reported in some series; treat as a reported association, not a mechanism.
Assessment
Diagnosis — the post-delivery examination
Diagnosis is overwhelmingly clinical and depends on a structured examination performed after every vaginal birth — not only those that "look bad". The reason OASIS is under-diagnosed is that examiners stop at the visible vaginal tear. The disciplined sequence is:
- Adequate analgesia and lighting. Inspect the entire perineum, vagina and the apex of any tear or episiotomy.
- Per rectum (PR) examination — mandatory. Insert a finger into the anal canal and a thumb into the vagina (or visualise directly) and palpate the sphincter while asking the woman to squeeze (if regional block allows). Feel for the characteristic gap or "torn drawstring" of the EAS and the paler, firmer IAS beneath the pink anal mucosa.
- Grade the injury using the classification above, documenting the percentage of EAS thickness and whether the IAS and mucosa are breached.
- Re-examine for a buttonhole rectal tear separate from the sphincter.
If there is any doubt, escalate to a more experienced operator before suturing — a tear closed as second-degree when it is in fact 3a is a missed OASIS. Training all accoucheurs (including midwives) to perform the PR examination is an explicit recommendation and the highest-yield single intervention for raising detection rates.
Anatomical and adjunctive assessment
Endoanal ultrasound is the reference standard for occult sphincter defects and is used in specialist follow-up and research, but it is not required for the intrapartum diagnosis and is not generally available at district level in South Africa. The intrapartum diagnosis stands on the clinical PR examination. Where suspicion of associated injury exists (e.g. extensive fourth-degree with rectal involvement), assess for and exclude a higher rectal/vaginal communication that would constitute a developing fistula.
History at follow-up
When a woman presents later, take a continence history (urgency, passive soiling, flatal vs solid incontinence, the validated symptom-severity tools used in colorectal practice), ask about perineal pain and dyspareunia, and examine the perineum and sphincter. This downstream presentation is the cost of a missed or poorly repaired primary injury.
Management
The verb of this objective is manage, so the weight sits here. Management spans prevention, emergency/primary repair, and definitive/follow-up care.
1. Prevention (antenatal and intrapartum)
- Perineal protection at crowning. A controlled, slow delivery of the head with a "hands-on" technique and verbal control of the woman's pushing to ease the head out between contractions reduces tear rates. This is the heart of perineal-protection.
- Selective, correctly-angled mediolateral episiotomy in high-risk situations (particularly instrumental delivery in a nullipara); routine episiotomy is not protective and is not recommended. Avoid the midline episiotomy.
- Warm perineal compresses in the second stage have evidence for reducing OASIS.
- Judicious instrumental delivery — appropriate choice of instrument and avoidance of difficult mid-cavity rotational delivery where a safer route exists. Integrate with complicated-labour and instrumental-delivery.
- Antenatal counselling and the "OASI care bundle" philosophy — a structured intrapartum bundle (risk awareness, manual perineal protection, mediolateral episiotomy where indicated, and systematic PR examination after birth) is the modern preventive framework.
Figure J10.2 — OASIS prevention and detection bundle linking risk scan, controlled crowning, mediolateral episiotomy, warm compresses and mandatory post-birth PR examination before closing a tear as second-degree.
2. Emergency / primary repair — the drill
Although OASIS is not a haemorrhagic emergency, the repair is a time-critical, environment-critical procedure that must be done right the first time. Treat the recognition-and-referral step as the emergency action:
OASIS REPAIR DRILL
- STOP and call for help — this is repaired by, or under the direct supervision of, an appropriately trained clinician, in an operating theatre with good lighting and assistance, not on the labour-ward bed under a delivery lamp.
- Anaesthesia — regional or general; adequate relaxation is essential to see and approximate the sphincter ends.
- Repair in theatre, in order, mucosa → IAS → EAS → perineal muscle → skin.
- Antibiotics — broad-spectrum cover at the time of repair (intra-operative ± a short course), classically including anaerobic cover, to reduce wound infection and breakdown that would precipitate fistula.
- Document the grade, technique, suture material, and the operator.
Technique detail:
- Anal/rectal mucosa (fourth-degree): repaired with a continuous or interrupted fine absorbable suture; knots tied in the anal lumen are traditionally avoided.
- Internal anal sphincter (IAS): identified as the pale structure and repaired separately with interrupted sutures by end-to-end approximation — explicitly identifying and repairing the IAS is a key quality step.
- External anal sphincter (EAS): for a full-thickness (3b, 3c, 4th) tear, repair by either overlapping or end-to-end technique — the major trial evidence shows no significant difference in outcome between the two at long-term follow-up, so the operator uses the method they are trained in; for a partial-thickness 3a tear, an end-to-end repair is used. A monofilament (e.g. polydioxanone) or a braided absorbable suture is used for the sphincter, with knots buried beneath the superficial perineal muscles to reduce suture-migration discomfort.
3. Post-operative / definitive management
- Antibiotics: complete the prescribed broad-spectrum course (anaerobic cover) to prevent infection-related breakdown.
- Laxatives: prescribe a course of an osmotic/stool-softening laxative (e.g. lactulose) — and avoid constipating agents — so the first stools do not strain the fresh repair. (Standard teaching; confirm agent and duration against your local protocol.)
- Analgesia: regular simple analgesia; avoid constipating opioids where possible.
- Catheterisation as needed for voiding, and monitoring for urinary retention.
- Physiotherapy: referral for pelvic-floor muscle training.
- Follow-up: review at ~6–12 weeks in a dedicated perineal/continence clinic where available, with a continence symptom enquiry. Endoanal ultrasound and anorectal manometry guide management of women with persistent symptoms or those considering a future vaginal birth, in specialist settings.
- Counselling about future delivery: a woman with a previous OASIS who is asymptomatic with normal investigations may be offered a subsequent vaginal birth; those who are symptomatic or have abnormal endoanal ultrasound / manometry should be counselled toward elective caesarean, with shared decision-making. Document the discussion. Cross-reference the mode-of-birth reasoning in vbac for the general approach to counselling about delivery after a prior complication.
Figure J10.3 — OASIS repair drill from recognition and theatre escalation through layer-by-layer mucosa, IAS, EAS and perineal repair, with post-operative protection, follow-up and future-birth counselling.
South African context
- The National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024) is the SA source of truth for intrapartum and perineal care; align local repair protocols to it and to facility levels of care.
- Detection at the periphery is the bottleneck. A large share of normal vaginal births in SA occur at district hospitals and Midwife Obstetric Units. The registrar's leverage is in training midwives to perform the routine post-delivery PR examination and to refer suspected OASIS to a clinician/facility that can repair in theatre — rather than the injury being closed as a second-degree tear and presenting later as incontinence or fistula.
- Resource realities: theatre access, suture availability (per the EML), and endoanal ultrasound for follow-up vary by level; the principle — repair in theatre, by a trained operator, with antibiotics and laxatives, then refer for follow-up — holds regardless of resource setting.
- HIV: a high local seroprevalence means wound healing and infection risk warrant attention; ensure antibiotic cover and follow the woman's ART per the SA HIV/ART consolidated guidance. Connect with hiv-in-pregnancy.
- Recurrent or missed injuries contributing to maternal morbidity feed into the Saving Mothers (NCCEMD) quality-of-care narrative even though OASIS is not itself a direct cause of maternal death — the avoidable-morbidity lens applies.
Red flags / pitfalls
- Missing the injury entirely by skipping the PR examination — the cardinal error. Every vaginal birth gets a rectal examination.
- Mis-grading — failing to identify and separately repair the IAS, or calling a 3a a second-degree tear. The IAS is the commonest layer overlooked.
- Missing a buttonhole rectal tear with an apparently intact sphincter — a setup for rectovaginal fistula.
- Repairing on the labour-ward bed under poor lighting and analgesia instead of in theatre — leads to inadequate approximation and breakdown.
- Using a midline episiotomy — propagates to the sphincter; use mediolateral.
- Constipating the patient post-repair (opioids without laxatives, or omitting stool softeners) — straining disrupts the repair.
- Omitting antibiotics — infection drives breakdown and fistula.
- Failing to counsel and document the future-delivery plan, leaving the next accoucheur without a record of a high-risk perineum.
- Wound breakdown, increasing pain, fever or purulent/faecal discharge post-repair → urgent senior review and assessment for infection or fistula.
Evidence anchors
- RCOG Green-top Guideline No. 29 — The Management of Third- and Fourth-degree Perineal Tears (OASIS). Primary guideline for classification, repair technique (overlap vs end-to-end equivalence, IAS repair, suture choice), antibiotics, laxatives, follow-up and future-delivery counselling.
- SA National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024) — South African intrapartum/perineal care source of truth and levels-of-care framework.
- NICE NG235 — Intrapartum care (2023) — perineal care, episiotomy and post-delivery assessment within routine intrapartum practice.
- RCOG Green-top Guideline No. 26 — Assisted Vaginal Birth — instrumental-delivery risk and mediolateral episiotomy in the high-risk delivery (links to instrumental-delivery).
- WHO Labour Care Guide (2020) — supportive intrapartum framework relevant to second-stage management.
- Saving Mothers / NCCEMD — SA maternal quality-of-care context.
- Notes on hedged facts: the OASI care bundle, warm-compress benefit, the ~60° mediolateral angle, the 4 kg macrosomia threshold, specific suture choices (polydioxanone), and the lactulose laxative regimen are stated as standard RCOG GTG 29 / intrapartum teaching but exact thresholds and agents should be confirmed against the current GTG 29 and local SA protocol before quoting numerically in an exam answer.